Mechanical apparatuses which move similarly to human beings by making use of electric or magnetic actions are called “robots”. The term “robot” is said to be derived from a Slavic word “ROBOTA (slave machine)”. In our country, robots came into widespread use at the end of 1960s, and many of them were industrial robots such as manipulators and transfer robots used in factories for the automatic, unmanned production operations.
In recent years, progress has been made in the research and development of legged mobile robots modeled after animals which walk upright on two feet, such as human beings and apes, for their body mechanisms and motions, and they are generally expected to be used for practical purposes. Although attitude control and walking control for legged mobile robots which walk upright on two feet are difficult to achieve compared to crawler-type, four-legged, or six-legged robots, they have advantages in that they are able to perform various kinds of moving operations such as walking on rough surface, including uneven ground and ground with obstacles, and moving on discontinuous surfaces, for example, climbing up and down stairs and ladders.
Legged mobile robots modeled after human beings for their body mechanisms and motions are called “human-shaped” or “humanoid” robots. Humanoid robots are used to assist people, that is, to help human activities in living environments and in other various areas of daily life.
The significance of the research and development of human-shaped, or humanoid, robots can be considered from, for example, the following two points of view.
One point is related to human science. Through the processes of manufacturing robots having structures similar to lower and/or upper limbs of human beings, inventing controlling methods for the robots, and simulating walking motions of human beings, mechanisms of the natural motions of human beings including walking can be elucidated from an engineering point of view. The results of such research will greatly contribute to the development of various other fields of research relating to human motion mechanisms such as ergonomics, rehabilitation engineering, and sports science.
The other point is related to the development of robots which serve as partners for people and assist them in their lives, that is, robots which help human activities in living environments and in other various areas of daily life. This type of robots must learn how to adapt itself to people having different personalities and to various environments while learning motions and manners from people in various aspects of life, and to keep increasing the functionality thereof. Therefore, it is believed that forming robots in a human shape, that is, making the shape or structure of the robots the same as that of human beings, is effective for smooth communication between people and robots.
For example, in the case in which a user (worker) tries to teach a robot a way to pass through a room while avoiding obstacles which should not be stepped on, it is much easier for the user to teach a biped walking robot having the same form as the user than to teach a crawler-type, four-legged, or six-legged robot. In addition, it must also be easier for the robot to learn from the user if the robot has the same form as the user (refer to, for example, “Control of Biped Walking Robot” written by Takanishi in <Koso> No. 25, April 1996, published by Kanto Branch, Society of Automotive Engineers of Japan, Inc.).
Most workspaces and living spaces of human beings, which walk upright on two feet, are designed in accordance with their body mechanisms and the behavioral patterns. Accordingly, there are many barriers for present mechanical systems using wheels or other driving devices as moving units to move in living spaces of human beings. In order for mechanical systems, that is, robots, to carry out various human tasks in place of people and to come into widespread use in people's living spaces, moving areas of the robots are preferably made to be the same as those of people. This is the reason why there are great expectations of putting legged mobile robots to practical use. Accordingly, in order to enhance the adaptability of robots to people's living environments, it is necessary to construct robots such that they walk upright on two feet similarly to human beings.
Humanoid robots may be used, for example, to carry out various difficult operations in industrial activities and production activities in place of people. For example, robots having a structure and functions similar to those of human beings may perform dangerous and difficult operations such as maintenance operations in nuclear power plants, thermal power plants, petrochemical plants, etc., transfer/assembly operations in factories, cleaning operations in high-rise buildings, and rescue operations at fire scenes, in place of people.
In addition, humanoid robots may also be used to “live together” with people in situations more closely related to people's daily life rather than to perform difficult operations in place of people. The goal for robots of this type is to faithfully reproduce the mechanism of whole-body coordinated motions which animals which walk upright on two feet, such as human beings and apes, naturally have, and to move as naturally and smoothly as human beings and apes. In modeling robots after highly intelligent animals such as human beings and apes, four limbs are preferably attached so that motions thereof will look natural. In addition, robots of this type are expected to perform motions having sufficient expressive power. Furthermore, they are required not only to faithfully follow commands input by a user but also to act in a lively manner in response to words and actions of people (such as praising, scolding, hitting, etc.). In this sense, entertainment-type humanoid robots modeled after human beings are rightly called “humanoid” robots.
Legged mobile robots including humanoid robots have a greater degree of freedom and accordingly have a larger number of joint actuators compared to other types of robots. More specifically, legged mobile robots have a large number of controlled objects in the system, so that the amount of calculation for attitude control and stable walking control increases exponentially. In addition, in biped walking robots, the center of gravity of their bodies is placed at a relatively high position and moves greatly during legged locomotion. Accordingly, bodies of biped walking robots of are naturally unstable (an existing area of the ZMP in biped walking robots is significantly small compared with that in four-legged walking robots), so that the amount of calculation for attitude control and stable walking control is extremely large.
In addition, it is almost impossible to perform stable, high-speed, real-time leg motion control independently inside a robot body. Accordingly, in general, legged robots perform walking motions which are determined in advance (for example, Japanese Unexamined Patent Application Publication No. 62-97006 discloses a control device for an articulated walking robot in which control programs are made simpler by using walking-pattern data items stored in advance and the walking-pattern data items can be reliably connected to each other).
A sequence of motions having a certain meaning performed by a robot is called a “behavior” or an action sequence. Action sequences are constructed by combining a plurality of motion pattern data items (also called “actions”), which indicate the time-sequential motion of each joint actuator installed in the robot body. More specifically, motion pattern data items, which indicate the time-sequential motion of each joint, are stored in a predetermined memory device, and when a predetermined action sequence is performed, motion pattern data items corresponding to that action are read out from the memory device and are played back on the robot body.
In order to generate various kinds of action sequences, basic or frequently-used motion pattern data items may be managed in a database and repeatedly used as data components. In the present specification, such motion pattern data items used as data components are referred to as “motion units”.
Basic motion units include, for example, a forward-motion start unit, a constant forward walking unit, a forward-motion stop unit, a rearward-motion start unit, a constant rearward walking unit, a rearward-motion stop unit, a leftward (or rightward) motion start unit, a constant leftward (or rightward) walking unit, a leftward (or rightward) motion stop unit, etc.
By combining several basic motion units, more complex motion patterns can be generated. For example, by combining the forward-motion start unit, the constant forward walking unit for N times of forward walking motions, and the forward-motion stop unit, it is possible to make a robot in a stationary state walk forward.
When various basic motion units are stored in a robot in advance, it is relatively easy to make the robot perform complex motions. In the above-described entertainment-type robots, it is strongly demanded that the motions thereof have sufficient expressive power. By changing combinations of basic motion units, various kinds of action sequences can be generated without increasing the number of motion units to be stored.
The basic motion patterns of a robot called motion units are generally described as data items which are arranged time sequentially, each data item indicating positions of each joint in a stationary state. In this case, whether or not two motion units can be connected is determined by taking into account only the continuity of the static positions of each joint (that is, static posture of the robot).
When a robot performs only static or relatively gentle motions, it may be able to perform stable motion by merely connecting motion units indicating only the rotation angles of each joint. However, when the robot performs fast motions wherein acceleration components cannot be ignored, the acceleration components serve as disturbances and affect the robot body so that there is a risk in that the stable motion will be degraded when motion units are connected. In addition, there is also a risk in that connecting points between motion units will be greatly limited due to the effects of the acceleration components. Motions in which the acceleration components cannot be ignored include a motion causing a state in which the ground reaction force is not applied to the robot body such as running motion or transitional motion before starting to run, a motion causing a state in which accelerated motion overpowers the acceleration of gravity and neither one of two feet is in contact with the ground, etc.